Multiple station, rotary indexing machine tool equipment is common in manufacturing installations and enjoys a high degree of commercial success. One of the most successful designs is disclosed in U.S. Pat. No. 2,947,188 issued to Bullard, et al. As described in Bullard et al., the multiple station machine tool comprises a circular base from which extends a fixed vertical column. The vertical column houses several machining stations consisting of tool slides that can move independently along vertical and horizontal axes. The base of the machine tool also supports a rotary worktable, or carrier, which is divided into multiple spindle stations each containing one or more workpiece carrying spindles. Machine tool equipment of this general description is commercially available as of the filing date hereof by DeVlieg-Bullard, Inc. of Westport, Connecticut, under the trade designation, "Type L Mult-Au-Matic Vertical Chucking Machines."
In operation, the worktable rotates around the vertical column indexing the spindles to successive machining stations. At each machining station, a particular operation is performed on the part carried by the spindle. Spindle speeds and tool feed rates at each station are set in reference to the operation performed. After a given operation is completed, the tools are retracted and the worktable rotates thus indexing the spindle stations so that different machining operations can be performed at the next machining station. Typically, a single drive motor provides the rotary power that drives all machine tool functions: worktable rotation, tool slide positioning and spindle rotation.
Despite the success of this highly pervasive design, multiple station machine tools of the type described above suffer from a number of shortcomings. First, the unified drive system mentioned above limits the types of machining operations that can be carried out by the machine. For example, it is not possible to vary the rates of horizontal and vertical tool slide movement given the arrangement of the tool slide drive mechanism, which makes it impractical to perform contouring operations. More significantly, the degree of precision obtainable by such a machine tool is substantially limited by the nature of the machine, its size and its mechanical complexity.
In a typical Mult-Au-Matic-type machine tool system, the actual position of each spindle can vary relative to the machine tool as the workpieces are indexed to successive machining stations. These positional variations come in two varieties. First, the exact location of each spindle within the worktable can vary from spindle to spindle. Second, the locking position of each spindle station indexed to a particular machining station can vary due to machine wear and operating conditions. As a result, the location of each spindle relative to the machining station can vary unpredictably. Notably, because the worktable diameter of a typical Mult-Au-Matic style machine can be six feet or more, the magnitude of these variations in position can be significant. The uncertainty of actual spindle position creates errors in the machining operations which have previously limited the machining tolerances achievable by an unmodified Mult-Au-Matic machine tool.
Due to the great popularity of this machine tool design, several attempts have been made to provide improvements. One such attempt is described in U.S. Pat. No. 4,351,096 to Depweg et al. The enhancements described in Depweg et al. include a modified machining station having independent drive motors for both tool slide positioning and spindle rotation. The individual drive motors of this modified station are connected to a computerized numerical control ("CNC") system which can control the motor speeds to provide variable speed operation. In addition, the CNC system can be programmed during machine installation to compensate for the fixed positional variation of each spindle within the surface of the worktable.
Although the improvements described in Depweg et al. provide improved machining tolerances (up to 0.0001 inch), they also suffer from a number of significant drawbacks. First, the use of separate drive motors to control spindle rotation and horizontal and vertical tool slide motion adds mechanical complexity and cost to the system. Second, the placement of these drive motors, and particularly the placement of the tool slide motor above the feed works platform of the machine tool, makes it difficult to upgrade an existing machine tool in a retrofit installation. Finally, no mechanism is provided for measurement and compensation for the dynamic variations in spindle station locking position.
As noted above, the variation in locking position of each spindle station is a non-repeatable error and can be expected to vary unpredictably during machine operation. Therefore, in order to compensate for such variations, the actual spindle position must be dynamically measured. Prior multiple station machine tool systems, including prior enhancements to conventional Mult-Au-Matic-type machines, fail to teach a structure for providing such error measurement and compensation.
Therefore, it will be desirable to provide a multiple station machine tool that is capable of improved machining accuracy. Moreover, it would be desirable to provide enhancements to the popular Mult-Au-Matic-type machine tool that provide such improved accuracy, wherein the enhancements could be added to the machine tool in a retrofit installation.